Molding finely divided sinterable material

ABSTRACT

Dispersions of sinterable powder, for example, of metal or high-melting-point non-metal such as silicon, or a ceramic substance such as silicon carbide, silicon nitride or alumina, are hot injection-molded using a dispersion of the sinterable material in a binder which, like the dispersion, is solid at room temperature and fluid at the temperature of molding, forming a molded product of uniform density. The binder employed contains polystyrene, which forms the predominant weight proportion of the binder, and in lower weight proportion, an organic compound, e.g., diphenyl carbonate, which is solid at room temperature, is soluble in the molten polystyrene, and is more soluble than the polystyrene is in a lower aliphatic alcohol or ketone. After molding, the binder is removed in non-solid form as vapor, or by dissolution in a solvent, or by oxidation to volatile materials, to leave a porous structure of uniform density.

This invention relates to the molding of finely divided sinterablematerial, such as a powder of metal or high-melting-point non-metal(e.g. silicon) or ceramic substance such as silicon carbide, siliconnitride or alumina.

The invention provides a method by which a dispersion of sinterablematerial can be injection-molded under moderate pressure to a productwhose shape is of considerable complexity, the product being not only ofuniform density but also such that the binder employed can be readilyremoved from it, so as to obtain a porous structure with density stilluniform (though, of course, lower) and with shape essentiallyunaffected. This in turn enables subsequent sintering to be carried outwith no substantial change in shape from that of the direct product ofinjection molding.

The method of the invention depends on the use of a novel binder of ourdiscovery, which has the necessary property of being solid at roomtemperature and fluid at the elevated temperature of injection molding.The binder comprises (i) a solid polystyrene, which forms thepredominant weight ingredient of the binder, and (ii) in lower weightproportion, an organic compound which is soluble in the moltenpolystyrene and is more soluble than polystyrene in a lower aliphaticalcohol or ketone, examples of which are methanol, ethanol, propanol,isopropanol, acetone and methyl ethyl ketone.

Preferably the organic compound has a melting point of at least 50° C.Preferred organic compounds are those whose molecular structure includesan aromatic ring, for example diphenyl carbonate (melting point about80° C.).

According to the invention there is provided a method which comprisesinjection-molding at elevated temperature a dispersion of finely dividedsinterable material in a binder, to obtain a molded product of uniformdensity, both the dispersion and the binder being solid at roomtemperature and fluid at the temperature of molding, said bindercomprising (i) polystyrene, which forms the predominant weightproportion of the binder, and (ii) in lower weight proportion, anorganic compound which is solid at room temperature, is soluble inmolten polystyrene and is preferentially soluble in a lower aliphaticalcohol or ketone; and removing said binder in non-solid form from themolded product, to leave a porous sinterable structure of uniformdensity.

The solid dispersion may additionally include a wax and/or a dispersant.It may also include an ethylene/vinyl acetate copolymer suitably in anamount 0.5-2.5^(w) /o of the dispersion to improve the rigidity of themolded product while the binder is being removed from it in non-solidform. In the latter expression we include removal by sublimation, bydissolution in a liquid solvent such as one of those (methanol etc.)previously referred to, and by controlled oxidation to form volatileproducts, e.g. benzoic acid, carbon dioxide and water vapor.

According to a preferred feature, the binder is removed from the moldedproduct in two stages, in the first of which the major part of theorganic compound is removed as vapour at elevated temperature; andsubsequently the polystyrene, with any residue of the organic compound,is removed at further-elevated temperature.

According to an alternative preferred feature, the binder is removedfrom the molded product in two stages, in the first of which the majorpart of the organic compound is removed by dissolution in a solventliquid; and subsequently the polystyrene, with any residue of theorganic compound, is removed at elevated temperature.

The solid dispersion may conveniently by prepared by mixing the finelydivided sinterable material with a solution of the polystyrene and theorganic compound in a relatively volatile organic solvent (preferably ofboiling point below 100° C.), and evaporating said solvent from themixture thus formed.

Preferred proportions of the various ingredients of the dispersion areas follows:

    ______________________________________                                                                       %                                              Ingredient    Main Function    By Weight                                      ______________________________________                                        1.  Sinterable material                                                                           --               70-95                                    2.  Polystyrene   Major Binder Component                                                                           2-10                                     3.  Diphenyl Carbonate                                                                          Minor Binder Component                                                                           2-10                                     4.  Wax           Minor Binder Component                                                                         0.5-5                                      5.  Dispersant    Dispersion of Powder                                                                           0.5-5                                                        in Organic Medium                                           6.  Plasticizer   Viscosity Modifier                                                                             0.2-2                                      7.  Ethylene vinyl                                                                              Improves Structural                                                                              0-2.5                                        Acetate Co-polymer                                                                          Rigidity During Binder                                                        Removal                                                     ______________________________________                                    

Molding is preferably carried out with a screw-type injection molder.

The invention is further illustrated by the following examples.

EXAMPLE 1

This example illustrates the application of the invention to thepreparation of a porous sinterable structure from silicon powder ofparticle size below 20 microns (mean particle size 5 microns).

A solid-form dispersion was prepared from the following ingredients:

    ______________________________________                                        Ingredient             % By Weight                                            ______________________________________                                        1.     Silicon Powder      81.0                                               2.     Polystyrene         6.8                                                       mol wt (M.sub.w) =                                                            150,000-300,000                                                               T.sub.G = 98° C.                                                3.     Diphenyl carbonate  4.7                                                       m. pt = 80° C.                                                  4.     Wax                 2.6                                                       Ethylene Bis Stearoyl Amide                                            5.     Surfactant          3.0                                                       Polymerized Fatty Acid                                                 6.     Plasticizer         0.9                                                       Di Butyl Phthalate                                                     7.     Ethylene/Vinyl      1.0                                                       Acetate Copolymer                                                             Melt Index: 60 dg/min                                                         VA content = 40 wt %                                                                              100.0                                              ______________________________________                                    

The ingredients 2, 3 and 5-7 were dissolved in methyl ethyl ketone(MEK), and a fine powder of the wax (4) was added to the resulting MEKsolution. The resulting low-viscosity liquid thus obtained was gentlychurned with the silicon powder in a sigma blade mixer at ambienttemperature. A free-flowing homogenous crumble was obtained from theresulting mixture by distilling off the MEK under reduced pressure.

The crumble was fusible to a pseudoplastic suspension of a viscosity 600Pa.S measured at a shear rate of 1000 s⁻¹, at a temperature of 120° C.It was subsequently molded at that temperature in a standard screw-typemachine [Negri Bossi NB90 or Arburg 270M-350-90].

The resulting products, which were of uniform density and robust enoughto be handled in the usual way, were then treated by one of two methodsin order to remove the binder in non-solid form (i.e. as vapor, liquidor gas) and leave a porous sinterable structure of uniform density.

a. The products were heated at 50°-70° C. under reduced pressure (1 mm)to cause the diphenyl carbonate (mp 80° C.) to sublime without causingthe molded product to distort. Once the bulk of the diphenyl carbonatehad been removed, the molded products were heated in a forced aircirculation oven at 20° C./hr to 400°-450° C. to oxidise the polystyreneto volatile material including benzoic acid, carbon dioxide and watervapor.

b. The products were immersed in methanol for 24 hours at ambienttemperature, to dissolve at least 65% of the diphenyl carbonate content(i.e. to remove it in liquid form), and were then heated to 420° C. at20° C./hour to oxidize the polystyrene and residual diphenyl carbonateas in a.

If the procedure of the above example is carried out without the minorproportion of diphenyl carbonate in the binder, then a considerablyhigher pressure is required to injection mold and molding has to beperformed at a much higher temperature (190° C.). Additionally more thantwice as long a time is needed for removal of the binder--a removalwhich has to be effected entirely by thermal degradation.

The porous products resulting from the above procedures which had goodstrength and rigidity were heated to 1360° C. in a nitrogen atmosphereover a period of 38 hours to yield reaction bonded silicon nitride witha final density of 2.40 g/cc.

EXAMPLE 2

Following generally the procedure of Example 1, a solid-form dispersionof the following composition was prepared from a stainless steel powderof particle size below 44 microns (mean particle size 11 microns):

    ______________________________________                                        Ingredient         % By Weight                                                ______________________________________                                        1.      Stainless Steel Powder                                                                       93.0                                                           (316L)                                                                2.      Polystyrene    2.5                                                    3.      Diphenyl Carbonate                                                                           2.2                                                    4.      Wax            1.0                                                    5.      Surfactant     1.1                                                    6.      Plasticizer    0.2                                                                           100.00                                                 ______________________________________                                    

The crumble obtained generally as in Example 1 was injection molded thistime at 100° C in a standard screw-type machine as in Example 1.

The molded products obtained were immersed in a stirred solution ofindustrial methylated spirit for 120 hours at ambient temperature todissolve at least 95% of the diphenyl carbonate content, and were thenheated to 450° C. at 15° C./hour in a stream of 75% H₂ /25% N₂, held atthis temperature for three hours and finally heated at 100° C./hour to750° C. in the same atmosphere. The porous, easily handleable structuresthus obtained exhibited good shape retention, and were sintered in aconventional manner at a temperature of 1280° C. to yield a product of94% theoretical density.

I claim:
 1. A method of preparing a porous sinterable structure ofuniform density comprising the steps of:(a) injection-molding atelevated temperature a dispersion of finely divided sinterable materialin a binder, to obtain a molded product of uniform density, both thedispersion and the binder being solid at room temperature and fluid atthe temperature of molding, said binder comprising (i) polystyrene,which forms the predominant weight proportion of the binder, and (ii) inlower weight proportion, diphenyl carbonate which is solid at roomtemperature, is soluble in molten polystyrene and is more soluble thanpolystyrene in a lower alkyl aliphatic alcohol or ketone; and (b)removing the binder in a two step process in non-solid form from themolded product by (i) vaporizing or dissolving a major part of thediphenyl carbonate, and thereafter (ii) removing any residue of thediphenyl carbonate and the polystyrene by heating at an elevatedtemperature, to leave a porous sinterable structure of uniform density.2. The method according to claim 1, in which the solid dispersion ofsintered material and binder additionally contains a wax.
 3. The methodaccording to claim 2,in which the wax is ethylene bis stearoyl amide. 4.The method according to claim 1, in which the solid dispersionadditionally contains an ethylene/vinyl acetate copolymer.
 5. The methodaccording to claim 1, in which in step (i), the major part of thediphenyl carbonate is removed as a vapor at an elevated temperature andsubsequently in step (ii), the polystyrene, with any residue of thediphenyl carbonate, is removed at a further elevated temperature.
 6. Themethod according to claim 1, in which in step (i), the major part of thediphenyl carbonate is removed by dissolution in a solvent liquid orvapor and subsequently in step (ii), the polystyrene, with any residueof the diphenyl carbonate, is removed at elevated temperature.
 7. Themethod according to claim 1, in which the solid dispersion is preparedby mixing the finely divided sinterable material with a solution of thepolystyrene and the diphenyl carbonate in a volatile organic solvent,and evaporating the solvent from the mixture thus formed.
 8. The methodaccording to claim 1, in which injection molding is carried out with ascrew-type injection molder.
 9. The method of making a shaped article,which comprises sintering a porous sinterable structure of uniformdensity obtained by the method of claim 1.